Alloy tool steels (JIS G4404) for hot dies typical of which is SKD61, and sintered ceramics have heretofore been used as materials for members of injection assemblies of die casting machines which are brought into contact with melts of aluminum, zinc and like nonferrous alloys, for example, for members such as plunger sleeve, piston, tip and gate sleeve.
The injection members made of the above-mentioned alloy tool steel are susceptible to corrosion due to contact with molten aluminum, zinc or like metal. Especially, the plunger sleeve is liable to corrosion and also to abrasion due to repeated sliding movement of the piston, therefore has a short life and requires much care for maintenance. Further rapid corrosion of such members which involves dissolving out of the material into the molten metal to be cast contaminates the melt to impair the quality of the casting.
The injection members made of sintered ceramic material are highly resistant to corrosion and abrasion but have the drawback of being inferior in impact resistance.
Accordingly, composite sintered materials have been proposed which have a mixed-phase structure comprising titanium or titanium alloy and a ceramic which are excellent in corrosion resistance (see, for example, Unexamined Japanese Patent Publications HEI 3-142053 and HEI 4-247801).
However, these composite sintered materials still remain to be improved although being excellent in corrosion resistance and abrasion resistance.
We have found that the insufficient strength of the composite sintered material is attributable to the fact that the microstructure thereof is low in homogeneity and involves uneven presence of ceramic particles as clustered. FIG. 4 shows the structure. The white portions are the metal phase, and the black portions are clusters of fine ceramic particles.
The composite sintered material of titanium-ceramic particles has a mixed-phase structure of low homogeneity because the titanium powder used as a material for sintering is larger in particle size than the ceramic powder mixed therewith as another material. Stated more specifically, the ceramic powder to be used for sintering has extremely small particle sizes of several micrometers (e.g., up to 5 micrometers), whereas titanium powders usually available are as coarse as about 20 to about 30 micrometers if smallest in size. Accordingly, even if the two materials are uniformly mixed together before sintering, the 15 sintered material obtained is in a mixed-phase state in which titanium grains are surrounded by fine ceramic particles.
Referring to FIG. 4 again which shows the mixed-phase structure, it is seen that ceramic particles (black portions) are unevenly present and form a network of clusters along grain boundaries of the titanium phase (approximately equal in grain size to the titanium powder used as the material).
On the other hand, in the case where the ceramic powder used for preparing the mixture to be sintered is generally as coarse as the titanium powder, it is possible to eliminate the uneven presence of ceramic particles in the form of a network of clusters surrounding titanium grains, but the presence of coarse ceramic particles entails the drawback of failing to form a dispersion of improved uniformity and a sintered material of compacted structure.